Eocene amber provides the first fossil record and bridges distributional gap in the rare genus Robsonomyia (Diptera: Keroplatidae)

Until now, the genus Robsonomyia was represented by two extant species: R. reducta Matile & Vockeroth, 1980 from North America and R. sciaraeformis (Okada, 1939) from Asia. This paper presents the first fossil members of the genus Robsonomyia, which is also the first record from Europe. Two new fossil species from Baltic amber are described: R. baltica Pełczyńska, Krzemiński & Blagoderov, sp. nov. and R. henningseni Pełczyńska, Krzemiński & Blagoderov, sp. nov.. The presence of fossil Robsonomyia spp. on the European continent suggests Holarctic distribution of the genus in the past. We also discuss possible pathways of its intercontinental dispersion.

Robsonomyia is distinguished from other members of the tribe by the shape of Sc vein which ends on Rb, instead of terminating in the costa 14 .Robsonomyia is currently represented by only two extant species, with a geographically disjunct distribution.R. reducta Matile & Vockeroth, 1980 is found in western North America, specifically in California (USA), and British Columbia (Canada) 14 .The other species, R. sciaraeformis (Okada,  1939), is native to East Asia, with occurrences in Japan, particularly in Sapporo, Hokkaido Island 15 .In this paper, we aim to shed light on the distributional history of Robsonomyia by incorporating new insights obtained from the discovery of their fossil representatives preserved in the Baltic amber.Type material: Baltic amber inclusion No NHMD-300551a (Fig. 1A); holotype (male) preserved in 19 × 13 × 4 mm piece of amber (Fig. 1B); paratypes No NHMD-300551b, No NHMD-300551c (two females) (Fig. 1C, D) preserved in 17 × 6 × 5 mm piece of amber (Fig. 1E).

Systematic Palaeontology
Diagnosis: Sc very short, weakens considerably just before ending in Rb vein; vein R 1 terminates distinctly before M 1+2 forking into M 1 and M 2 ; vein M 1 3.5 × longer than M 1+2 ; vein R 2+3+4+5 strongly arched from half of its length towards anterior of the wing; vein M 3+4 joins with m-cu almost opposite Rs; gonostylus as long as gonocoxites, conical, curved mesally just before the end, pointed at apex.
Type material: Baltic amber inclusion No NHMD-39356 (Fig. 4A); preserved in 10 × 8 × 5 mm piece of amber (Fig. 4B).Diagnosis: vein R 1 terminates distinctly after M 1+2 forking into M 1 and M 2 ; vein R 2+3+4+5 strongly arched from half of its length towards anterior of the wing; M 1 3.1 × longer than M 1+2 ; between Rs and m-cu the basal part of M 3+4 is present, in the form of transverse basale (tb vein); gonostylus cylindrical, wide at the base, strongly narrowed and gradually arching inwards in apical half, pointed at apex.Description: body 2.6 mm long, wing length 1.8 mm, antennae 1.3 mm long; Head (Fig. 5A): subspherical, wider than long; eyes large, well separated; membranous area separates ocellar sclerite from frons; large distinct cerebral sclerite present; three ocelli forming equilateral triangle.Antennae (Fig. 5B): about 0.7 × of wing length, about 0.5 × as long as body; scapus slightly shorter than broad, 1.4 × wider than flagellomeres; pedicel with bulbous apical part, as broad as scape; flagellum 14 segmented; flagellomeres cylindrical, densely covered with short setae, elongated, approx.1.2 × longer than broad; terminal flagellomere slightly elongate, evenly tapered to rounded apex, 1.5 × longer than proximal flagellomeres.Palpi with three visible maxillary palpomeres.Wings (Fig. 5D, E): 2.4 × longer than wide, membrane hyaline without macrotrichia and without any visible markings; costa ending close after M 1 vein reaches wing margin; Sc short, reaching Rb approximately at level of m-cu reaching Cu; R 1 ending just after mid-length of anterior margin of the wing, distinctly after level of M 1+2 forking into M 1 and M 2 ; R 2+3+4+5 strongly arched anteriorly, second half of the vein runs almost in parallel with C; cross-vein m-cu 0.9 × as long as the radio-medal fusion (frm), between Rs and m-cu basal part of M 3+4 is present (transverse basale = tb); Rs distinct, oblique, ending on the level of tb; Mb absent; M 1+2 2,3 × longer than frm, ending at the level of half the distance between where A and Cu terminates; Cu and A 1 reaching wing margin; A 2 absent.Thorax (Fig. 5C): about as high as long, scutum densely covered with long and thick hairs; anepisternum and katepisternum bare; mediopleural suture almost straight and subvertical; mesepimeron and laterotergite bare, mediotergite round and bare; haltere longer than first abdominal segment.Legs (Fig. 6D-F): fore coxa sparsely covered with long hair-like setae, mid and hind coxa with only a few setae; femora densely covered with short, robust setae; fore tibia with single apical spur, anterior tibial comb absent, mid and hind tibia with two equal length spurs, more robust and longer on hind tibia.Abdomen: densely covered with long hairs, all eight segments visible, I segment short, segments II-IV approx.same length, following segments gradually decreasing in length, segment VIII retracted into VII; male terminalia (Fig. 6A-C): tergite IX almost as long as broad, subcylindrical, slightly narrower at apex; gonocoxites massive, fused ventrally, almost straight at the apical margin ventrally; gonostylus cylindrical, wide at the base, strongly narrowed in its second half, curving gradually, pointed at apex; aedeagus and the associated internal structures not visible.

Discussion
The decision to include the newly discovered species in Robsonomyiini was primarily based on the structure of the head.A unique apomorphy present in both species were observed, a membranous area separating the ocellar sclerite and the frons (Fig. 2A).In both species, the space between the isolated sclerites is narrow, but this may be a consequence of preservation and deformation during the fossilisation process.In addition, the cerebral sclerite is large and posteriorly extended, but not strongly defined and divergent from the head, which is typical of the tribe.Other features common to Robsonomyiini were found in the thorax (mediopleural suture is non-sinusoidal and subvertical), legs (lack of anterior tibial comb) and male genitalia (gonocoxites are fused and almost straight at the apical margin ventrally, whereas in Macrocerini they are usually distinctly concave).While the placement in the genus Robsonomyia itself was determined by the wing venation with a very characteristic shape of the Sc vein ending on Rb instead of terminating on the costa (diagnostic feature of the genus) 10,14 .The fossil Robsonomyia species described here significantly improve our knowledge of the biogeographic history of the genus, expanding its current distribution.The Baltic origin of the specimens was confirmed with transform infrared spectroscopy analysis.The FTIR spectra showed distinctive features characteristic of the Baltic amber, including the "Baltic shoulder" observed in the range 1190-1280 cm −1 , accompanied by a strong absorption peak at 1170 cm −1 (Fig. 7) 18 .
This proved presence of Robsonomyia in Europe during the Eocene allows us to hypothesize that the current disjunct distribution is the relict of an earlier wider Holarctic distribution (Fig. 8).This pattern of occurrence is reminiscent of numerous other groups that were once widespread in the northern middle latitudes during the initial stages of the Tertiary period.For example, the disjunct pattern of distribution between Eastern Asia and Eastern Palaearctic and Nearctic exists in at least 65 genera of flowering plants that went extinct in the western Eurasia most likely due to orogenic events and climate change at the end of the Tertiary and during the Quaternary 19 .The classic example of this distribution pattern is represented by ginseng (Araliaceae: Panax) 20 .Among insects, the scorpionfly family Panorpodidae (genus Panorpodes) represents a similar case to Robsonomyia.Extant panorpodids are currently found only in eastern Asia (Japan, Korea and China) and North America but four species have been discovered in Baltic amber 21 .In the family Keroplatidae, there are genera that are now exclusively Nearctic, but for which amber records indicate a past Holarctic distribution.This is the case of Palaeoplatyura Meunier, 1899 (Keroplatinae) and Hesperodes Coquillett, 1900 (Macrocerinae) 1 .
By comparing representatives of Robsonomyia between each other we can observe morphological differentiation (Table 2) present in the wing venation (Fig. 9), the length of antennae and structure of genitalia (Fig. 10).A feature common to all species is the subcostal vein that joins the Rb vein although in R. baltica sp., it weakens considerably at the apex (Fig. 9A).Greater variation is observed in the medial sector.The presence of the tb crossvein (transverse basale) is observed only in R. henningseni sp.nov.(Fig. 9B, Table 2).In the other species, vein tb is absent, which is an apomorphic characteristic.Moreover, absence of the basal part of the M 3+4, separating vein from the m-cu cross vein is observed in R. reducta (Fig. 9C, Table 2).Notably, the anal sector of this species displays an additional apomorphy as anal vein (A 1 ) does not reach the edge of the wing (Table 2).
Furthermore, there are notable differences in the structure of the male genitalia (Fig. 10, Table 2).The gonostylus can be long and exhibit a simple, cylindrical shape with a pointed apex (as seen in R. henningseni sp.nov.and R. baltica sp.nov.; Fig. 10A, B) or be short and flattened with a wide and blunt tip (as observed in R. reducta; Fig. 10C1, C2).The gonostyli of Sciaroidea, in their most basic layout, are simple, cylindrical tubes, that are closed at the apex 10,22 .Accordingly, any modifications such as shortening or thickening can be considered as apomorphic features 10,23 .The finding of two species of genus Robsonomyia in the Eocene Baltic amber will certainly support future phylogenetic analyses, both in terms of dating the clades and in terms of enriching diagnostic features.The species found in Baltic amber suggest that Robsonomyia appeared at the latest in the Eocene.The current distribution of the genus is relictual and could be a vicariant pattern resulting from the subdivision of an ancestral wide distribution range followed by extinction in the western Palearctic.Alternatively, Robsonomyia could have dispersed out of the western Palearctic across Eurasia and further to east North America across one of the Beringian land   bridges (BLB) that have intermittently connected these continents, or west across the North Atlantic, which was less extensive in the Eocene across, e.g., the Thulean route (TR) or earlier by De Geer route (DGR) (Fig. 8) 24,25 , or both.Additional information from fossils in North America and/or East Asia and a dated phylogeny for Macrocerinae is essential to test the probability of these different scenarios.

Material and methods
The specimens examined were found in the Baltic amber, a fossil resin of Eocene origin with an age span from the Lutetian to the Priabonian (47.8-33.9Ma) 9 .However, the precise age of the amber remains unknown.The main reason is secondary redeposition; the amber has been transported and dispersed across the Northern European Plain, due to inter alia marine transgression and glaciers during the Pleistocene 26,27 .Consequently, the amber is not found in its original sedimentary context, and its stratigraphic history remains elusive 28 .Additionally, Baltic amber lacks radiogenic isotopes with long-term half-lives, preventing the direct application of radioisotopic dating methods for precise age determination 26 .The age of Baltic amber has been a subject of extensive debate, leading to the implementation of various methods that have yielded different results.For instance, glauconite dating has suggested a middle Eocene (Lutetian) origin for the amber, whereas microfossil dating has indicated a late Eocene (Priabon) timeframe 29 .
In this study, a total of four keroplatid inclusions were examined (2A, C, D and 5C).These inclusions consisted of one male and two females found within a single piece of amber NHMD-300551 (Robsonymia baltica sp.nov.), as well as one additional male from a separate piece NHMD-39356 (Robsonomyia henningseni sp.nov.).The specimens are housed in the collection of the Natural History Museum of Denmark (NHMD) in Copenhagen.Piece NHMD-300551 was cut in two during preparation for study, one piece containing the male holotype (NHMD-300551a) and the other containing two female paratypes (NHMD-300551b, NHMD-300551c).To enhance the visibility of the inclusions, the amber pieces underwent preparation, involving cutting, grinding, and polishing.To validate the authenticity of the fossil material, a Fourier transform infrared spectroscopy (FTIR) analysis was conducted.The analysis employed a Nicolet iS5 FTIR spectrometer, which was equipped with a diamond crystal attenuated total reflectance (ATR) attachment.The recorded spectra have been archived within the database of ISEA PAS as recommended for museum material by Zakrzewska et al. (2020) 16 .
Photographic documentation was performed using a Leica M205 C stereomicroscope equipped with a Leica DMC5400 camera.Focus stacks were acquired and processed in Leica Application Suite X (LAS X) (leicamicrosystems.com/products/microscope-software/p/leica-las-x-ls).Drawings were generated by tracing photographs in CorelDRAW 2018 software (coreldraw.com/en/product/coreldraw).Additionally, a distribution map was created using the SimpleMappr online generator (simplemappr.net)and then modified using CorelDRAW 2018.The terminology used in this publication follows Matile (1990)